Gas flow monitor



Feb. 6, 1 968 s. D; KLYCE -3,368,2 12

I GAS FLOW MONITOR Filed Dec. 8, 1964 United States Patent 3,368,212 GASFLOW MONITOR Stephen D. Klyce, Harvard, Mass; Stephen D. Klyce, Jr.,executor of said Stephen D. Klyce, deceased, assignor to J. H. EmersonCompany, Cambridge, Mass, a corporation of Massachusetts Filed Dec. 8,1964, Ser. No. 416,777 7 Claims. (Cl. 340239) ABSTRACT OF THE DISCLOSUREA system for supervising the respiration of a patient includes a brassblock having a main threaded bore for connection to a respirationsensing device and two transverse channels which intersect the mainbore. A 0.053 inch I.D. nylon tube in each channel supports athermistor. One tube is blocked and the other tube is exposed to gasflow through the main bore. The two thermistors are connected in abridge circuit which controls, via a silicon controlled rectifier, anaudible alarm oscillator and a pilot light to operate the alarm andlight whenever gas flow due to the patients breathing falls below apreestablished value. A power failure alarm operates the oscillator onlyshould system power fail.

This invention relates to electronic condition detection apparatus andmore particularly to electronic apparatus for monitoring flowconditions.

It is frequently desirable to monitor gas fiow in a manner whichintroduces minimal interference with the gas flow condition beingsupervised. For example, in the care and treatment of infants or otherpatients who may need respiratory support, it is not always possible tohave a nurse in attendance. In connection with the treatment of suchpatients, it would be desirable to provide an arrangement whereby therespiration of the patient could be continuously monitored, and shouldan abnormal condition develop, an alarm would be generated which wouldcall for prompt assistance to the patient by a nurse, for example. Suchapparatus should be capable of accurately monitoring the flow conditionwhile not interfering with the air flow in the breathing circuit of thepatient. As each patient must be individually monitored, the monitoringapparatus should be simple and relatively economical, while capable ofadjustment to supervise the relatively Wide range of gas flow conditionswhich may be encountered in such applications.

Accordingly, it is an object of this invention to provide novel andimproved apparatus for monitoring the breathing conditions of a patient.

Another and more general object of the invention is to provide novel andimproved gas flow detection apparatus which is reliable and effective inoperation and yet relatively simple and economical in construction.

In accordance with the invention there is provided a gas flow monitoremploying a thermally responsive gas flow sensing element. The sensingelement is connected in electrical circuit in one leg of a bridge typeof sensing circuit. A corresponding compensating thermally responsiveelement is preferably connected in the adjacent leg of the bridgesensing circuit and provides compensation for changes in ambientconditions to which the gas flow detector apparatus is exposed. Onediagonal of the bridge circuit is energized from a suitable DCenergizing source, and an output signal derived froin the other diagonalof the bridge circuit is applied to a gate controlled asymmetricallyconductive device (silicon controlled rectifier). In the particularembodiment described hereinafter in detail, the output signal from thebridge circuit is applied between the gate electrode and the cathodeelectrode of the rectifier device. Facility for adjustment of the bridgecircuit enables compensation for the pressure of the gas flow conditionto which the sensing element is to be exposed. Output or alarm devicesare connected directly in series with the gate controlled device and apower source, and when the bridge output signal applied to the gatecontrolled device permits that device to conduct, the output device ordevices are energized and signal the sensed change in flow condition.The apparatus is sensitive and responds to gas flow changes ofrelatively small magnitude while sensing only a fraction of the totalgas flow required for adequate breathing, for example. In addition, theapparatus readily may accommodate a warning circuit which operates whenthe supply power fails, for example. Apparatus constructed in accordancewith the invention is simple to adjust and operate. It is relativelyeconomical to manufacture and is reliable in operation. Gas flow ratesas low as a few cubic centimeters per minute may be monitored atpressures as low as 0.001 inch of water. While the invention hasparticular utility for monitoring the breathing conditions of patients,it also has general utility in monitoring of other gas fiow conditions,as in industrial operations.

Other objects, features and advantages of the invention will be seen asthe following description of a particu lar embodiment thereofprogresses, in conjunction with the drawing, in which:

FIG. 1 is a schematic diagram of the gas fiow detector constructed inaccordance with the invention; and

FIG. 2 is a diagrammatic view of the mounting of the sensor elementemployed in conjunction with the system indicated in FIG. 1.

The gas flow monitor apparatus shown in the schematic diagram of FIG. 1employs a dual secondary step-down transformer 10 which has its primarywinding 12 arranged for connection to a volt source via terminals 14,16. Switch 181 controls the application of energy to transformer 10, andthat transformer, when energized, produces a twelve volt signal as anoutput from one secondary winding 20 and a twenty-four volt signal fromits other secondary winding 22. The twelve volt signal is applied to afull wave bridge rectifier circuit which includes four diodes 24. Asmoothing capacitor 26 is connected across one diagonal of that bridgecircuit, The output signal from that diagonal of the bridge istransmitted through resistor 28 and regulated by Zener voltage regulatordiode 30 for application to the monitor bridge circuit that includes twoidentical temperature variable resistance elements 32, 34, two fixedresistance elements 36, 38, and a potentiometer 40. The rectified DCsignal from the bridge rectifier circuit is connected to one diagonal ofthe monitor bridge and an output signal voltage is extracted from theother diagonal of that bridge.

Both variable resistance elements 32, 34 are thermistors, and theirresistance characteristics change as a function of the temperature towhich they are exposed. The thermistors are self-heated to above anyforseeable ambient temperature so that the temperature of the airflowing by the sensing head portion does not affect accuracy. Element 32is the gas flow sensing element and is directly exposed to a fraction ofthe gas flow to be monitored, while element 34 is a compensating elementand is connected in the bridge circuit so that its change in resistance,in response to changes in ambient conditions to which the monitoringapparatus is exposed, compen sates for corresponding changes in theresistance of monitor element 32 produced by ambient changes.

The monitor thermistor S2 is disposed in a portion of the gas streamwhich is to be supervised by means of the mounting arrangement shown inFIG. 2. That mounting includes a thermally conductive block 50 of brasswhich has a main threaded longitudinal bore 52 that extendsapproximately two-thirds the length of the block, and two transversechannels 54 56 which intersect the main bore adjacent its bottom. Acoupling 58 is threadedly secured in bore 52 and has an extensionportion 60 to which a rubber tube 62 or other suitable conduit isattached. Tube 62 is connected to the bypass gas flow extractingelement-a needle disposed in the gas fiow line to be supervised, forexample. Nylon tubes 64, 66 are inserted in channels 54, 56,respectively, and in each tube is disposed one of the thermistors,thermistor 32 being disposed in tube 64 and thermistor 34 being disposedin tube 66. Each nylon tube has an ID. of 0.053" and receives thethermistor bead that is 0.043" in diameter. The two thermisto'rs thusare supported in symmetrical insulated relation on block 50. Tube 64 isunblocked and forms a part of the flow passage for the portion of thegas flow to be sensed, but tube 66 has plugs 68 in each end. Thus, thetwo thermistors are identically mounted closely adjacent heat sink block50, but only the sensing thermistor 32 is exposed to gas flow, whichflow may take place in either direction along the path indicated bydashed lines.

The illustrated apparatus is useful for monitoring gas fiow produced bybreathing of a patient, and in such monitoring only a small fraction ofthe total gas flow produced by the patient is monitored so that thepossible interference with breathing is nonexistent. The monitor may becoupled to a pneumograph which encircles the patients chest or may beinserted directly in the patients breathing circuit if such breathing isbeing supported mechanically. For example, a hose through which the gasis flowing may be pierced with a needle which is connected to tube 62for conducting a minor portion of the air How in the hose past themonitoring thermistor 32 to cause a temperature drop therein. With themounting of sensor element 32 as shown in FIG, 2, less than 0.1% of theair used by the patient is lost.

This monitor bridge has an output circuit that includes series resistor70 and parallel resistor 72. (For increased sensitivity these resistorsmay be omitted and the value of resistor 38 changed to 470 ohms.) Thejunction 74 of those two resistors is connected to the gate electrode 76of silicon controlled rectifier 78, while the other line 80 of thebridge output circuit is connected to the cathode electrode 81 of thesilicon controlled rectifier. In series with the silicon controlledrectifier 78 is the twenty-four volt secondary 22 of transformer and agroup of parallel devices which include an alarm light 82, an audiblealarm oscillator device 84 sold under the trademark Sonalert, and asmoothing capacitor 86 to improve the operation of the audible alarm. Apilot light 88 is connected across secondary winding 22.

In operation of the apparatus, when switch 18-1 is closed, power isapplied via secondary and the diode bridge rectifier to the monitorbridge. (Pilot lamp 88 is energized by secondary 22 as a visualindication that the apparatus is energized.) Monitor element 32 isdisposed in a bypass of the gas flow to be monitored, and potentiometeris adjusted until the audible alarm 84 is first sounded. The device isthen attached to a pneumograph or other air flow source. Any air flowpast the sensor 32 will cause its temperature and resistance to drop,thereby decreasing the voltage applied to the gate 76 and causing thesilicon controlled rectifier 78 to cease firing and shut off the alarm.If the air flow by the sensor 32 decreases, the sensors resistance andtemperature increases, causing the voltage to gate 76 to increase andthe silicon controlled rectifier to fire and sound the alarm 84.Potentiometer 40 is adjusted to a position of greater circuitinsensitivity if the flow and/or pressure to be measured is greater. Formonitoring intermittant fiow such as breathing, the potentiometer 40 isadjusted to a sufiiciently insensitive point so the alarm does not soundbetween breaths. The thermistors 32, 34 preferably are also large enoughto retain sufficient heat to cause a delay before the silicon controlledrectifier fires, so that it does not fire between breaths. In otherapplications where gas flow is in one direction, the gas flow ispreferably through block 50 prior to passing thermistor 32 so as toreduce the effect of transient variations in temperature of the gasflow.

A power failure alarm may be connected in the circuit through theprovision of relay coil 90 which controls contacts 92 and a battery 94.Switch 18-2, which is operated simultaneously with line switch 18-1,completes the circuit between battery 94 and oscillator 84. However,relay coil 90 is immediately energized and opens contacts 92 so that theoscillator circuit is not energized from battery 94. Should power fail,relay 90 will be deenergized and contacts 92 close so the oscillator 84is energized and produces an audible alarm. It will be noted that thebattery 94 is connected in circuit only under power failure conditionsand the only circuit component connected to the battery 94 on suchconditions is the audible alarm oscillator 34 so that the battery drainis minimal. (In the preferred embodiment the control for potentiometer40 is mounted on the same shaft as the control for switches 18-1, 18-2and this coupling is indicated by dashed line 96.)

While a particular embodiment of the invention and modifications thereofhave been shown and described, still other modifications thereof will beobvious to those of ordinary skill in the art, and therefore, it is notintended that the invention be limited to the disclosed embodiment or todetails thereof, and departures may be made therefrom within the spiritand scope of the invention as defined in the claims.

What is claimed is:

1. Gas flow monitoring apparatus comprising a resistance bridge circuithaving four legs and defining two diagonals,

an electrical resistance element disposed in each leg of said bridge,one of said resistance elements having a. resistance characteristic thatvaries as a function of the temperature of the resistance element,

a second one of said resistance elements being identical to said oneresistance element and connected in a different leg of said bridgecircuit to compensate for changes in ambient temperature to which saidfirst resistance element is exposed,

a mounting structure for said two identical resistance elementscomprising a thermally conductive mass having a gas flow passagetherethrough for connection to the gas flow to be monitored,

means supporting said one resistance element in said gas flow passage inclose proximity to but spaced from said thermally conductive mass, and

means supporting said second one of said resistance elements outsidesaid gas fiow passage but otherwise in a similar position relative tosaid thermally conductive mass,

cir-cuit means for applying an energizing signal across one diagonal ofsaid bridge,

a gate controlled asymmetrically conductive device having first andsecond main circuit terminals and a gate terminal,

means connecting said gate terminal and one of said main circuitterminals across the other diagonal of said bridge, and

an indicator device connected between said first and second main circuitterminals.

2. The apparatus as claimed in claim 1 and further including adjustmentmeans in said bridge circuit for varying the electrical resistance intwo of said bridge legs to control the conduction of said asymmetricallyconductive device.

3. Gas flow monitoring apparatus comprising a gate controlledasymmetrically conductive device having first and second main circuitterminals and a gate electrode,

an output device for providing an indication of the sensed gas flowcondition,

means connecting said main circuit terminals and said output device inseries,

a monitor bridge circuit coupled to said gate electrode of saidasymmetrically conductive device,

said monitor bridge circuit having four legs and including a gas flowsensing element connected in one leg of said bridge circuit, saidelement having an electrical characteristic that varies as a function ofthe gas flow past said sensing element,

a second element having an electrical characteristic identical with saidgas flow sensing element connected in a second leg of said bridgecircuit to compensate for changes in ambient temperature to which saidgas flow sensing element is exposed,

means to energize said monitor bridge circuit, and

a mounting structure for said two identical elements comprising athermally conductive mass having a gas flow passage therethrough forconnection to the gas flow to be monitored,

means supporting said gas flow sensing element in said gas flow passagein close proximity to but spaced from said thermally conductive mass,and

means supporting said second element outside of said gas flow passagebut otherwise in a similar position relative to said thermallyconductive mass.

4. Gas flow monitoring apparatus comprising a transformer having aprimary winding and two secondary windings,

rectifier means coupled to one of said secondary wind mgs,

a gate controlled asymmetrically conductive device having first andsecond main circuit terminals and a gate terminal,

an output device for providing an indication of the sensed gas flowcondition,

means connecting said first and second main circuit terminals, saidoutput device, and the other of said secondary windings in series,

a monitor bridge circuit coupled between said rectifier means and saidgate terminal of said asymmetrically conductive device,

said monitor bridge circuit having four legs and including a firstresistance element connected in one leg of said bridge circuit, saidfirst element having a resistance characteristic that varies as afunction of the temperature of the resistance element, a secondresistance element identical to said first resistance element connectedin said bridge circuit to compensate for changes in ambient temperatureto which said first resistance element is exposed, and

a mounting structure for said first and second resistance elementscomprising a thermally conductive mass having a gas flow passagetherethrough for connection to the gas flow to be monitored,

means supporting said first resistance element in said gas flow passagein close proximity to but space-d from said thermally conductive mass,and

means supporting said second resistance element in correspondingposition relative to said thermally conductive mass.

5. The apparatus as claimed in claim 4 wherein each said resistanceelement support means is a plastic tube,

the tube supporting said first resistance element defining a portion ofsaid gas flow passage, and

the tube supporting said second resistance element being blocked toprevent gas flow past said second resistance element.

6. The apparatus as claimed in claim 4 and further including anauxiliary power source connected for energizing only said output device,

relay means including contacts in circuit with said auxiliary source,and

means responsive to the energization of said transformer to operate saidrelay means and move said contacts to open the circuit between saidauxiliary source and said output device.

7. The apparatus as claimed in claim 4 wherein each of said identicalresistance elements is a thermistor.

References Cited UNITED STATES PATENTS 2,439,331 4/ 1948 Bean 340 -3332,499,806 3/ 1950 Wouk et al 340237 3,258,655 6/1966 Pinckaers 3 l7148.5

JOHN W. CALDWELL, Primary Examiner. MAX L. LEVY, NEIL C. READ,Examiners. D. MYER, Assistant Examiner.

